The Harrower is the Super Champion and can only be summoned in Felucca. In
order to summon it, all six of the other spawn Champions must be defeated. Each
time a Champion is defeated, they will leave behind a special item: a hued skull.
The color and type of the skull depends on the Champion you slay.

Once you slay a Champion, you must take the hued skull through the dark moongate
that will appear. The gate will lead to a 'star room' that contains an altar
for each Champion. Once all six skulls have been placed on their respective
altars, the Harrower will spawn in a random dungeon in Felucca, and another
dark moongate, leading to that dungeon, will appear on the altar

You can attain new titles from defeating the Harrower:

Super Champion Titles

Defeated Harrower +

Title

0 Champions

Spite of Evil

1 Champion

Opponent of Evil

2 Champions

Hunter of Evil

3 Champions

Venom of Evil

4 Champions

Executioner of Evil

5 Champions

Annihilator of Evil

6 Champions

Champion of Evil

But beware this creature, for it is twice the menace of any other Champion!

Rewards

Aside from the items and gold that you can get from fallen creatures, the rewards
from killing a Champion or the Harrower can certainly be worth all the effort
put into completing the challenge.

Not only can you gain new titles, but in Felucca you can also be rewarded various Scrolls of Power by participating in fighting the Champion spawn - lesser scrolls to those that only fight the minions, while
greater scrolls will be given to those that actually play a part in taking down
the Champion itself.